1. Field of the Invention
This invention relates to a light-emitting device using a thin film transistor for a drive circuit and a pixel portion.
2. Related Art
In a semiconductor display device formed by using a glass substrate at low cost, the required area for the periphery of a pixel portion (a frame region) for mounting is increased with the increase of screen resolution. Accordingly, the miniaturization of the semiconductor display device tends to be prevented. Therefore, it has been considered that there is a limitation in a method of mounting IC formed by a single crystalline silicon wafer on a glass substrate. Hence, a technique of forming integrally an integrated circuit including a drive circuit over one substrate with a pixel portion, that is, so-called system on panel, has been attracted attention.
A thin film transistor formed by a polycrystalline semiconductor film (polycrystalline TFT) has advantages that the mobility thereof is two orders of magnitude higher than that of a TFT formed by an amorphous semiconductor film, and a pixel portion and a drive circuit around the periphery thereof of a semiconductor display device can be integrally formed over one substrate. However, there are problems that the process becomes complicated due to crystallization of a semiconductor film, yields becomes reduced, and the cost becomes increased compared to the case of using an amorphous semiconductor film.
In the case of performing laser annealing that is used generally for forming a polycrystalline semiconductor film, energy density required for improving crystallinity should be secured. Accordingly, throughput in the process of crystallization is declined and the crystallinity by the edge-neighborhood of a laser beam is varied due to that the longitudinal length of the laser beam has limitations, and so the size of a substrate has limitations. The variation of the energy of laser light causes the variations of the crystallinity of a semiconductor film. There is a problem that it is difficult to laser anneal uniformly a subject.
However, a TFT in which a channel formation region is formed by an amorphous semiconductor film can obtain electric field effect mobility only of approximately from 0.4 to 0.8 cm2/Vsec. Therefore, the TFT can be used as a switching element in a pixel portion, however, the TFT is unsuitable for a drive circuit required for high speed operation such as a scanning line drive circuit for selecting pixels, or a signal line drive circuit for supplying a video signal to the selected pixel.
Especially, in the case of an active matrix light-emitting device among semiconductor devices, at least two transistors, that is, a transistor serving as a switching element for controlling the input of a video signal and a transistor for controlling the supply of current to the light-emitting element are provided in a pixel. The transistor for controlling the supply of current to the light-emitting element is preferably to have higher ON current than that of the transistor used as a switching element. Therefore, the further improvement of the mobility of a TFT in a pixel portion is an important challenge for a light-emitting device.